This invention relates to substantially unused environmental protective coatings; and, more particularly, to such coatings that include at least one region deviating from a selected specified range.
Components operating in the gas path environment of a power generating apparatus, for example a gas turbine engine, are subject to significant temperature extremes and degradation by an oxidizing and corrosive environment. It has been a practice in that art to apply at least one of oxidation resistant, corrosion resistant, and thermal barrier coatings to exposed surfaces of such components to protect them from the operating environment. At the same time, the protective coating affords the opportunity to improve engine efficiency by increasing operating conditions. Typical coatings include aluminum diffused into a component surface or an underlying layer of a protective or bond coat for example of the well known MCrAl type, with or without an outer coating of a ceramic type thermal barrier coating. In the MCrAl type coating, the xe2x80x9cMxe2x80x9d represents at least one of Fe, Ni, and Co, and the alloy can include other elements such as Y. These coatings are well known, and have been widely used and described in connection with gas turbine engines.
It is intended that coated components be manufactured in selected, specified ranges including ranges and levels for coating chemistry and coating physical condition, for example coating thickness. At times, processing difficulties encountered in isolated steps in the manufacturing process have resulted in a coating that, in at least one identified coating region, is too thin and/or has an incorrect chemistry. Correction of such a defect in a coating, frequently having a portion that has been diffused into a component surface, includes removing the defective coating and reapplying an acceptable coating, or scrapping the component. Both of such actions are costly in effort, time delays, etc. In addition, removal and reapplication of coatings can thin a component wall, for example an airfoil wall, to near or below specified minimums. This either requires scrapping of the component or it limits future repairability.
In another situation, a manufacturing specification has been changed after component manufacture or otherwise it is desired to upgrade the coating on an existing, unused component. For example, this can occur when it is desired to upgrade a relatively simple diffused aluminide coating to one including a performance enhancing secondary element such as at least one of Pt, Rh, Pd, Cr, Si, Hf, Zr, and Y, used for such purposes in coatings in the turbine engine art.
Provision of a method for enhancing a substantially unused, existing protective coating without requiring removal of the coating can be beneficial for a variety of reasons including reduced cost of component manufacture and time delays.
The present invention, in one form, provides a method for enhancing on an article a substantially unused protective coating, including aluminum, to a selected coating range. The method comprises first evaluating the unused coating to identify at least one substantially unused coating region that deviates from the selected range by being deficient in at least one of chemistry and physical condition. The coating region is enhanced to the selected coating range by applying over the unused coating at least to the coating region at least one secondary coating element selected from Pt, Rh, Pd, Cr, Si, Hf, Zr, and Y. Then at least the secondary element is heated to diffuse the element into the unused coating to provide a treated coating region.